1. Field of the Invention
The present invention relates to a semiconductor laser device having self-aligned structure (SAS type semiconductor laser device), and more particularly, to this type of semiconductor laser device which is capable of high optical output power and also ensures high-reliability operation for a long term.
2. Description of the Related Art
A SAS type semiconductor laser device is a high optical-output laser device capable of simultaneously confining injected current and laser beam within the cavity, and is usually produced using a GaAs-based compound semiconductor.
FIG. 1 shows an exemplary layer structure A of a conventional semiconductor laser device of this type.
As illustrated, on a GaAs substrate 1 of the device are successively formed a buffer layer 2 of n-GaAs with a thickness of 0.5 xcexcm, a lower cladding layer 3 of n-Al0.3Ga0.7As with a thickness of 2.0 xcexcm, a lower optical confinement layer 4 of i-Al0.1Ga0.9As with a thickness of 50 nm, an active layer 5 consisting of a quantum well layer of In0.2Ga0.8As with a thickness of 7 nm and a barrier layer of Al0.1Ga0.9As with a thickness of 10 nm, and an upper optical confinement layer 6 of i-Al0.1Ga0.9As with a thickness of 50 nm. Further, an upper cladding layer 7a of p-Al0.3Ga0.7As with a thickness of 500 nm and a low refractive-index layer 8 of n-Al0.35Ga0.65As with a thickness of 0.5 xcexcm, which serves also as a current blocking layer, are formed on the upper optical confinement layer 6, and these layers are covered with an upper cladding layer 7b of p-Al0.3Ga0.7As with a thickness of 2.0 xcexcm. On the upper cladding layer 7b, a contact layer 9 of p-GaAs with a thickness of 0.5 xcexcm is formed. An upper electrode (not shown) is formed on the contact layer 9, and a lower electrode (also not shown) is formed on the opposite side, that is, the lower surface of the substrate 1.
In the illustrated layer structure A, the current blocking layer (low refractive-index layer) 8 has a channel 10 with a predetermined width formed therein as a current injection path continuous with the upper cladding layer 7a, thus forming a light-and-current confinement structure extending across the transverse (width) direction.
The width W of the channel 10, that is, the width of a portion thereof where the electric field distribution is highest within the channel, is determined in conjunction with the transverse mode control for oscillated laser beam. Specifically, in the case of the aforementioned semiconductor laser device of which the upper cladding layer 7a is made of p-Al0.3Ga0.7As and has a thickness of 500 nm or thereabout, the width W of the channel 10 is designed to be about 2.5 xcexcm in consideration of the cutoff width required to cause the device to operate in fundamental transverse mode while cutting off higher-order mode of laser beam oscillated at the active layer 5.
The device is produced in the following manner.
First, the buffer layer 2, the lower cladding layer 3, the lower optical confinement layer 4, the active layer 5 and the upper optical confinement layer 6 are successively formed on the substrate 1 by MOCVD or MBE. The active layer 5 is made of double In0.2Ga0.8As (7 nm-thick each) quantum wells separated by GaAs (10 nm-thick) barrier, and GaAs (20 nm-thick) optical confinement layers located both sides of quantum wells. Then, after the upper cladding layer 7a of about 500 nm thick is formed on the upper optical confinement layer, a layer 8xe2x80x2 as a layer to be formed a low refractive-index layer is formed on the upper cladding layer, thereby obtaining a layer structure A0 shown in FIG. 2.
Subsequently, the layer structure A0 is taken out of the crystal growing apparatus and subjected to photolithography and wet etching to form the channel 10 with a channel width W of 2.5 xcexcm in the layer 8xe2x80x2, thereby obtaining a layer structure A1 (FIG. 3) having the current blocking layer (low refractive-index layer) 8. Side faces 10a of the channel 10 formed by the wet etching are inclined under the influence of etching anisotropy, so that the channel broadens upward.
Then, the layer structure A1 is again placed in the crystal growing apparatus and the upper cladding layer 7b and the contact layer 9 are successively formed on the layer structure A1, thus obtaining a layer structure A2 shown in FIG. 4.
After upper and lower electrodes are formed on the layer structure A2, the structure is cleaved such that it has a cavity length of 800 xcexcm, thus obtaining the layer structure A shown in FIG. 1. A film with 5% reflectivity is formed on one cleaved face (front facet) S1, while a film with 92% reflectivity is formed on the other cleaved face (rear facet) S2, whereby the intended laser device is obtained.
The laser device produced in the above-described manner has a threshold current of 15 mA and has a maximum optical output, which is limited by kink, of about 350 mW in fundamental mode. The oscillation wavelength is approximately 980 nm.
The above laser device of which the low refractive-index layer 8 has a cutoff width of about 2.5 xcexcm is associated with the following problems.
If the optical output of the laser device is increased to meet the recent demand for higher optical output, an additional problem arises besides the generation of kink. Specifically, if the laser device is operated at an optical output of about 500 mW, the light density at the front facet S1 increases up to a level of as high as several tens of MW/cm2. Consequently, the front facet S1 is optically damaged, that is, COMD (Catastrophic Optical Mirror Damage) occurs, with the result that the laser device cannot be used thereafter.
A second problem arises because the semiconductor material constituting the low refractive-index layer 8 consists of Al0.35Ga0.65As and has high content of Al in order that the low refractive-index layer 8 may be lower in refractive index than the upper cladding layers 7a and 7b. Specifically, after the layer structure Al is formed, the surface (side faces) of the channel 10 formed in the low refractive-index layer 8 is inevitably exposed to the air before the upper cladding layer 7b is formed thereon, and thus can sometimes be oxidized by the oxygen in the air. If the surface of the channel 10 (low refractive-index layer 8) is oxidized, especially if the side faces of the channel 10 close to the active layer 5 are oxidized, the optical output of the produced laser device lowers in a short period of time when the device is continuously operated, with the result that the device fails to ensure long-term reliability.
An object of the present invention is to provide a SAS type semiconductor laser device which solves the above problems with SAS type semiconductor laser devices having the layer structure shown in FIG. 1, which scarcely develops COMD even if oscillated at an optical output of as high as 1000 mW or more, and which maintains high reliability for a long term.
To achieve the above object, the present invention provides a SAS type semiconductor laser device having an active layer, and a low refractive-index layer formed close the active layer and functioning also as a current blocking layer, wherein the low refractive-index layer includes a plurality of compound semiconductor layers made of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61), and the compound semiconductor layers have refractive indices thereof set such that the refractive index lowers with increasing distance from the active layer.
Specifically, in the semiconductor laser device, the compound semiconductor layers have contents of Al thereof set such that the content of Al increases with increasing distance from the active layer.
The present invention also provides a SAS type semiconductor laser device having an active layer, and a low refractive-index layer formed close the active layer and functioning also as a current blocking layer, wherein the low refractive-index layer includes a plurality of compound semiconductor layers made of Ga1xe2x88x92yInyAszP1xe2x88x92z (0xe2x89xa6yxe2x89xa60.5, 0xe2x89xa6zxe2x89xa61), and the compound semiconductor layers have refractive indices thereof set such that the refractive index lowers with increasing distance from the active layer.